1. Field of the Invention
The present invention relates generally to indicators used to monitor the humidity of an environment, and more particularly, to a humidity indicator which provides an irreversible indication that a predetermined humidity level has been reached while substantially delaying the reaction time of such an indicator.
2. Description of the Prior Art
When shipping or storing corrosion-succeptible equipment, such as electronic and military related equipment, it is desirable to know whether the interior of containers used to ship or store such equipment has been exposed to humidity levels sufficient to cause corrosion of components within such equipment.
Humidity indicators which reversibly change colors in response to the surrounding humidity level are generally known in the art. Such prior art reversible humidity indicators typically utilize cobalt chloride as the indicator material, and the prevailing level of humidity determines the color of the indicator material at any particular time. Military specifcations have been developed regarding the packaging, colors, an other features of such humidity indicators, for example, military specification number MIL-I-26860.
However, there is a need, particularly in military applications, to detect the occurrence of a predetermined level of humidity and to indicate the same without allowing the indication to reverse itself after humidity levels have dropped. Were a long term storage container, stored within a relatively warm climate, to develop an air leak, moisture could enter the container during periods of relatively high humidity in the vicinity of the container. Upon subsequent cooling of the container, admitted moisture may condense and cause corrosion of the equipment stored within the container. Subsequent warming and drying of the air external to and surrounding the container can subsequently draw out all of the moisture previously admitted into the container. Thus, a reversible type humidity indicator will fail to indicate the temporary presence of high humidity levels within the container for durations sufficiently long to cause corrosion.
In view of the above mentioned disadvantages of reversible type humidity indicators, irreversible humidity indicators have been developed and are known in the art. For example, U.S. Pat. No. 2,214,354 discloses an irreversible humidity indicating device wherein a deliquescent agent such as calcium chloride is mixed with a water soluble dye, and the mixture is disposed upon a porous surface such as a sheet of absorbent paper. Upon being exposed to a predetermined humidity level, the deliquescent agent liquifies and dissolves the dye; the dye solution is carried by capillary action into the porous surface or paper for producing a permanent and irreversible marking. The aforementioned patent further describes various deliquescent agents which may be alternately employed for indicating different humidity levels.
Shipping containers and long term storage containers for corrosion-succeptible equipment are usually packed with a desiccant to dehydrate the area enclosed by the container after it is sealed. Periodically, such containers are opened to recharge or replace the desiccant packed within each such container. After recharging or replacing the desiccant, the container is again sealed. Sufficient quantities of desiccant are typically packed within each such container to ensure that the space enclosed by the container is dehydrated to safe levels within approximately one hour or less after the container is seal However, in the event that the container is surrounded by relatively humid environment either during the initial operation of storing the equipment within the container or during subsequent operations to recharge or replace the desiccant within the container, the irreversible humidity indicator of the type disclosed by the aforementioned patent may be prematurely actuated due to the temporarily high humidity levels within the container both before the container is sealed and also during the interval after sealing while the desiccant initially dehydrates the interior of the container. For example, at 85 percent relative humidity, it may take only 15 to 30 minutes to activates such an irreversible humidity indicator. Such premature actuation requires removal of the irreversible humidity indicator initially present and replacement with a fresh irreversible humidity indicator.
Accordingly, it is an object of the present invention to provide an irreversible humidity indicator adapted to monitor humidity levels within long term storage containers or the like wherein the response time of the indicator is substantially delayed over that of presently known irreversible humidity indicators to facilitate the initial packaging of corrosion-succeptible equipment within such containers and to facilitate the subsequent opening of such containers to replace or recharge desiccant therein without prematurely actuating the indicator.
It is a further object of the present invention to provide such an irreversible humidity indicator adaptable to existing humidity indicator packaging and mounting techniques.
It is yet another object of the present invention to provide such an irreversible humidity indicator which may be manufactured easily and inexpensively.
These and other objects of the present invention will become more apparent to those skilled in the art as the description thereof proceeds.